The present invention is directed to linear motion devices and more particularly to such devices having gripper assemblies for linearly moving an element in stepwise fashion.
Linear motion devices are particularly adapted to move various elements in a linear direction to any desired position, such as elements used in controlling a complex chemical process involving frequently a high temperature environment or for controlling various elements of a complex machine tool. Also, the elements to be positioned may be located within a sealed pressure vessel which requires some type of seal where the linear motion device enters the sealed vessel, such as the elements used in controlling the power output of a nuclear reactor.
Linear motion devices forming the prior art having utilized latching or gripper assemblies which are actuated by suitable actuating means, such as a solenoid coil, to cause engagement or disengagement of the element to be moved. Generally, the gripper assembly engages the element and moves it a small increment in the direction of desired linear motion at which point the element is engaged by a holding means. The gripper assembly is then decoupled from the element, returned to its initial position, and is again coupled to the element. At that time the holding means is decoupled from the element and the gripper assembly and the element are moved an additional increment in the aforementioned direction. With some such systems of the prior art, provisions have been made to remove the load placed on the gripper assembly by the element before the gripper assembly is decoupled therefrom. These devices have also included means for disposing the element relative to the gripper assembly so that frictional engagement of the gripper assembly and the linear element during the coupling portion of its cycle does not occur. This has been found desirable since continuous frictional engagement between the gripper assembly and the element during its coupling and decoupling portions of its cycle produces a substantial amount of wear on the grippers. As a result, frequent replacement of the grippers has been required and thereby limits the use of such linear motion devices in applications wherein frequent maintenance and repair is impractical.
In the aforementioned prior art gripper type linear motion devices having load transfer features, the load transfer function was generally provided by the holding means which comprised a second gripper assembly. The second gripper assembly was adapted to be moved into an engagable position between adjacent projections on the element and then adapted to be raised to engage the element and lift the element to remove the load from the first gripper assembly. While the second gripper assembly held the element, the first gripper assembly was decoupled frictionlessly from the element, returned to its initial position, and then coupled again frictionlessly to the element. Lowering of the second gripper assembly then transferred the load back to the first gripper assembly and removed the load from the second gripper assembly to permit frictionless disengagement thereof. Accordingly, such devices did eliminate excessive wear on the grippers with the resulting mechanisms having a substantially longer life of operation.
In many uses of such devices, such as control element drive mechanisms for nuclear reactors, it is important that the linearly movable element be positioned as accurately as possible. Since these types of apparatus can only operate in discrete steps, this requires relatively small steps of operation so as to obtain the desired accurate positioning. Furthermore, the size of the steps is limited by the fact that dynamic forces due to magnetic action in moving a slug or armature to a new position increase as the length or distance of movement increases. Therefore, while rapid movement would be a desirable feature of these drives, it is limited by the size of the step which may be taken in each cycle. Further still, with the gripper type linear motion devices as described hereinabove, only a single linear motion step occurs during each cycle of operation--that is, only one gripper assembly operates to move the element linearly while the other gripper assembly merely acts to transfer the load to permit coupling or decoupling frictionlessly of the first gripper assembly.
While a greater number of stepping motions in a single cycle could be achieved with use of a greater number of gripper assemblies, this would be undesirable since it would result in a larger sized and more expensive linear motion device. Furthermore, it would require a greater number of solenoid or magnetic coils which are extremely expensive to manufacture, especially for use in high temperature applications. Further still, such devices would still require one gripper assembly which is operative only to provide a load transfer function and which does not also operate to move the element an increment in a direction of desired linear motion. Accordingly, this results in at least one solenoid coil not being used for any purpose other than transferring the load.
One prior art device having two gripper assemblies, shown in U.S. Pat. No. 3,626,493, does disclose the idea of providing two stepwise motions in a single cycle. Such a device provides for more rapid movement of the linear element, or alternatively for a decrease in the size of each of the steps while maintaining the same rate of movement of the element. However this device cannot provide a load transfer feature for both raising and lowering of the linearly movable element, or if a load transfer function is provided, the element can only be moved in one direction. As can be appreciated, especially with respect to control element drive mechanisms for nuclear reactors, incremental motion in either of two opposed directions is highly desirable.
It is to be noted in connection with all the prior art gripper type linear motion devices that the projections or teeth on the linearly movable elements are closely spaced from one another so that when the gripper assembly is moved into an engagable position between adjacent teeth, only a small gap exists between the gripper assembly and the adjacent teeth. While such an arrangement has produced successful operation for single step gripper type linear motion devices, it has not proven acceptable where two stepwise motions are performed in a single cycle by a minimum number of gripper assemblies each of which is operative to provide load transfer features.